Effectiveness of a Presynchronization Program Implemented on a Modern Dairy Facility By R. E. Thommen Dairy Science Department College of Agriculture CALIFORNIA POLYTECHNIC STATE UNIVERSITY San Luis Obispo 2010
ABSRACT The objective of this study was to determine the success of a presynchronization program implemented on a modern dairy facility. The study was conducted on a 1,400 cow dairy in the central valley of California. 86 cows were involved in the study during the summer months of June through September. Cows were either assigned in the synchronization program or placed in a control group based on the last digit of their identification. At 43 DIM, presync cows were injected with their first treatment of the synchronization program. Estrous cycles of the presync group were manipulated with treatments of Prostaglandin and GnRH. Treatments were applied so that cows had a predetermined breeding date between 60 and 81 days after calving. Detection of estrous was performed daily with tail paint removal being the primary method of detection for the control group. The control group was eligible for insemination once reaching the 60 day voluntary waiting period. First service conception rates for the synchronized and control groups were 31% and 37% respectively, while the 21 day pregnancy rate for synchronized animals were 19% and 14%. With a 5% difference, the presynchronization program appears to be more successful in achieving higher reproductive efficiency. ii
Contents LIST OF TABLES... iv LIST OF FIGURES... v INTRODUCTION... 1 LITITURE REVIEW... 2 MATERIALS AND METHODS... 8 RESULTS AND DISCUSSION... 12 CONCLUSION... 18 iii
LIST OF TABLES Table Page 1. Presync Group Data..16 2. Control Group Data... 18 iv
LIST OF FIGURES Figure Page 1. Presynch treatment schedule....9 2. 1 st service conception rate... 13 3. 21 day pregnancy rate... 14 4. DIM 1 st service presynch group... 15 5. DIM 1 st service control group...15 v
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INTRODUCTION With highly variable markets and decreased profit margins on today s dairy farms, modern dairies must aim to operate more efficiently than ever before. One of the most essential aspects of managing an efficient operation is cow reproduction. Reproduction on dairies has direct effects on many aspects of the operation. The profitability and future success of an operation are ultimately influenced by the herd s reproductive performance. With profit margins lower than ever before, dairies cannot afford feeding cows that remain open during late stages of lactation as milk production is hampered and their productive future is questionable. In an attempt to improve reproductive performance and reduce open cows during late stages of lactation, estrous synchronization programs have been adopted. In previous studies, estrous synchronization programs have increased heat detection rates, the main factor influencing reproductive success in an AI program. The purpose of this study was to determine the success of a presynchronization program implemented on a dairy under specific circumstances. Although synchronization programs have been proven successful for many dairies in past studies, they may not be practical or successful for certain facilities and management styles. The specific dairy where the study took place operated at overcapacity and used more conventional management practices. Past studies were recorded at intensively managed facilities under highly controlled environments. During this study, strict compliance was enforced with a systematic breeding program while other aspects of the dairy went unchanged.
LITITURE REVIEW Reproductive management is a critical factor in managing a dairy farm in today s increasingly competitive industry. The reproductive performance on dairies directly affects profitability on farms as it influences milk production per cow, availability of replacements, and culling rates. Artificial Insemination (A.I.) has been widely used as an essential tool for breeding dairy cattle over the last 50 years. A.I. has not only been successful in increasing a dairy s genetic merit, but in many cases also improving management on dairies. With the help of A.I., today s dairy cows are capable of producing nearly double the amount of milk they did just 50 years ago. Unfortunately the largest contribution to poor reproductive performance in most herds is the failure to accurately detect heats, which is detrimental to any A.I. program. Estrus detection rates in dairy herds are frequently lower than 50% ( Rabiee et al. 2005). With suppressed levels of efficiency for many herds, estrus detection is often at the forefront for improving reproductive performance. Estrus detection is an ongoing challenge for dairies as there are many reasons which contribute to poor heat detection rates on farms. Factors contributing to poor heat detection rates are often linked to environmental stress and poor management. Heat detection especially falls during periods of high temperatures as estrous behavior is less apparent due to environmental stress. Animal housing also plays a large role in heat detection as overcrowding and undesirable floor surfaces reduce a cow s ability to mount. Cows permanently housed in free stall facilities will likely not exhibit estrous to the same degree as cows housed in open lots. This is largely due to hard, slippery floors associated with frees stall facilities, preventing cows from mounting or standing during the onset of estrous. The main contributor to missed heats and inaccurate detection relies heavily on personnel detection. 2
Management accounts for 90 percent of the failure to detect estrous while the other 10 percent is the failure of cows to express estrous (Bilby and Chebel, 2009). The failure of cows to express estrous plays is roll in poor estrous detection. By 60 days in milk (DIM) 25% of lactating dairy cows are considered anovular or not cycling (Gumen et al., 2007). The primary cause of anovular cows is the presence of a cystic ovary. Studies show that 70 % of cystic cows are anestrous (Hutchinson,2008 ). Estrous synchronization programs often aid in the regression of ovarian and luteal cyst, especially if GnRH is incorporated in the protocol. In an attempt to improve heat detection rates, many dairy farms are adopting new methods such as activity monitors and hormones to manipulate a cow s estrous cycle. Heat detection is not the only factor hindering the performance on a dairy s reproduction. Efficient breeding of cows and heifers has become increasingly difficult as fertility has decreased due to a correlated negative response with milk yield (Olynk1 and Wolf, 2008). Other possible causes of poor reproduction might be caused from difficult calving or inadequate nutrition. Having an inadequate energy intake, especially in early lactation is likely to be detrimental to a cow s fertility. In this situation the cow will be using most of her energy for maintenance and production while leaving little for reproduction. On the other hand, excessive energy intake during late lactation and the dry period can cause fat cow problems which lowers reproductive efficiency in the next lactation (Smith and Chase, 2010). Manipulating a cow s estrous cycle to create a planned breeding date in advance is a method used to increase heat detection efficiency. Manipulation of the cow s estrous cycle is becoming a common practice in the dairy industry and is often referred to as estrous synchronization. There are many forms and variations of estrous synchronization programs that 3
are commonly used on farms to manipulate the estrous cycle. Estrous synchronization is accomplished by injecting, feeding, or impregnating hormones that affect a cow s reproductive cycle. The objective of a synchronization program is to initiate the development of a new follicular wave while promoting ovulation at the appropriate time. Knowing when a cow is expected to ovulate decreases the need for estrous detection. Having a predetermined breeding date reduces the chance of not observing a valuable estrous, leading to higher A.I. submission rates. In larger operations, groups are synchronized together which allows for the majority of inseminations to be done within a window of 2-3 days. This reduces the need to detect estrous on each individual animal, therefore labor for heat detection is reduced. These programs vary in their practicality, convenience, and effectiveness with regard to each specific facility and management style. From farm to farm, breeding programs vary tremendously ranging from the use of bulls to intensive synchronization programs. The majority of synchronization programs utilize Prostaglandin (PG 2a ) and Gonaditrophin releasing hormone (GnRH) as the key hormones to synchronize ovulation of the follicle. Prostaglandin has a luteolytic effect on animals that have an active Corpus Luteum (CL) during the time of the injection. Normal cycling cows will generally have a developed CL from days 6 through 17 of their cycle. Injecting cows with PG 2a that do not have a developed CL (days 1 through 7of their cycle) will have no effect. For this reason two separate injections are given 14 days apart for most synchronization programs to ensure all cows will be on the same stage of their cycle after the second PG 2a injection. Prostaglandin causes the Corpus Luteum to regress prematurely which is known to induce estrus within 2 to 7 days following an injection. Caution must be used when 4
administering this hormone as it will also regress the Corpus Luteum on pregnant animals, resulting in abortion of the fetus. GnRH administered intramuscularly triggers the release of endogenous LH or FSH from the anterior pituitary. The release of Lutenizing Hormone (LH) or Follicle Stimulating Hormone (FSH) causes the follicle to grow and mature while eventually causing ovulation. GnRH is used for treatment of cystic ovaries and estrous synchronization. Ovsynch is a synchronization protocol that utilizes these two hormones for follicular synchronization. GnRH is initially injected followed by an injection of PG 2α 7 days later. A second GnRH is given 2 days after the PG 2α injection. This protocol is successful for timed artificial insemination (TAI) as it both controls corpus luteum regression and initiates a new follicle wave. The initial GnRH acts to reset the cycle, inducing ovulation and promoting the growth of a new corpus luteum along with a new follicular wave. Once the new CL begins development, a shot of PG 2α 7 days later regresses that corpus luteum. 2 days following the PG 2α injection, a second GnRH shot will given to induce ovulation of the new follicle. All animals are scheduled for breeding 16-24 hours after the last GnRH injection. Although most cows will enter estrous during the expected timed artificial insemination period, some (5-15%) will enter estrous slightly before or after the expected date. This protocol requires 10 days from the initial injection until the time of insemination. Modified ovsynch or presynch is a variation of ovsynch in which cows are given 2 PG 2a injections 14 days apart before the initial GnRH is given to start the ovsynch portion. This program is especially effective in scheduling a breeding date for a cow s first service post calving. Although this protocol does require more time and resources, studies show an increase in pregnancy rates. In a meta-analysis comparing multiple breeding programs, Increased 5
pregnancy rates were detected when cows that were presynchronized before implementation of Ovsynch were compared with cows that had no presynchronization treatments (Rabiee et al. 2005). The probable cause for an increase in pregnancy rates is due to a higher number of presynchronized cows entering the ovsynch portion of the protocol at a more desired stage of the estrous cycle (Thatcher et al. 2001). Another contribution to an elevated pregnancy rate is the result of a more functional uterus. The first two injections have a healing effect, which cleans and conditions the uterus. Not only does the presynchronization program increase conception rates for cows inseminated during TAI, it allows for heat detection throughout all stages of the protocol. It is common for many reproductive management programs to breed all cows expressing heats after the second PG 2α. This method is desired by many as it reduces the days in milk for first breeding, while also reducing drug and labor expenses. Prostaglandin based programs are also an option for use as a heat detection aid. Amongst other programs, prostaglandin based protocols offer the most rapid results of estrous synchronization as cows may come into heat as soon as 3 to 4 days after the first injection of PG 2a. Cows coming into estrous 3 to 4 days following the first PG 2a injection are cows that have a functional corpus luteum at the time of injection. This group of cows will then be inseminated while the remaining cows not showing estrous will receive a second PG 2a shot 7 days after the initial injection. 3 to 4 days following the second injection the remaining cows should enter estrous and will then be inseminated. Unlike timed A.I programs, cows not exhibiting estrous should not be inseminated as PG 2a only affects the Corpus Luteum and not the follicle. 6
Natural Artificial Insemination breeding is based off of inseminated at observed estrous. With natural breeding there is no systematic approach to manipulate a cow s estrous cycle; therefore heat detection must be practiced cautiously. Heat detection aids must be used with this approach as there is no telling otherwise when the cow should enter estrous. A.I. submission rates are often lower as many heats go unobserved. In the same meta-analysis mentioned earlier, the natural breeding group had lower pregnancy rates compared to ovsynch programs (Rabiee et. al 2005). Although pregnancy rates are often lower due to insufficient heat detection, many studies have shown an increase in conception rates based on insemination from observed natural estrous. 7
MATERIALS AND METHODS The study was conducted on a 1400 cow dairy in Firebaugh, California. Cows were milked twice daily and were not receiving bst during the duration of the study. Animals were housed in a shaded free stall facility where they had access to exercise lots and groomed freestalls. Following parturition, all cows were grouped amongst other cows that had recently freshened (stocking density of 100%) for a length of two months. Following the 2 month stay in the fresh pen, cows were then moved without strategic grouping in pens with an average stocking density of 120%. The study was conducted from the summer months of June 29, 2010 to August 3, 2010. Two separate reproductive management groups were observed and compared. Both groups were housed together to avoid a bias study. Animals whose identification ended in even numbers were assigned to the experiment group and placed on a presynchronization program. There were 43 cows in the experiment group that were placed in the presynchronization program after 40 DIM. Animals whose identification ended in odd numbers were assigned to the natural breeding group, which acted as the control group for the experiment. This group was also composed of 43 animals. Both groups contained an equal number of animals in each lactation. All cows were housed together and locked once daily for heat detection and artificial insemination when necessary. Prostaglandin(PG 2a ) and Gonadotrophic Releasing Hormone (GnRH) were the two hormones used to synchronize the experimental group. The prostaglandin chosen for this experiment was Lutalyse (dinoprost tromethamine) made by the Phizer company.10cc of Lutalyse were injected in the hind leg with a 20gneedle for each treatment. The GnRH product 8
chosen was Cysterelin (Gonadorelin Diacetate Tetrahydrate) produced by the Merial Company. In the hind leg, 5cc s of GnRH were injected subcutaneously with a 20g needle for every dose. Working together these two hormones act to synchronize heat expression and cause ovulation. Every Tuesday from June 29, 2010 to August 3, 2010 a new cluster of even numbered animals were assigned on the presynch protocol. Cows that calved within a seven day period were assigned to a cluster where all cows within the cluster were treated simultaneously. During the study, 6 clusters composed of 5 to 15 animals were formed. Every Tuesday, cows ranging from 43 to 50 DIM were subcutaneously injected with their first treatment. Presync Treatment Schedule Sunday Monday Tuesday Wednesday Thursday Friday Saturday 1 2 3 4 5 Inject PG 2 α 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Inject PG 2 α BREED COWS SHOWING ESTROUS 20 21 22 23 24 25 26 27 28 Inject GnRH Inject Inject Timed AI PG 2 α GnRH Figure 1- Presync Treatment Schedule 9
The schedule of injections proceeds as follows: (43 DIM) day 0 PG 2a day 14 PG 2a day 28 GnRH day 35 PG 2a day 37 GnRH day 38 timed A.I. All injections were given in the morning during routine heat detection. Any cow that did not receive injections on schedule was removed from the study along with cows that died or were sold. The Voluntary waiting period was 60 DIM which was strategically planned 3 days after the second PG injection. All cows not exhibiting estrous 3 to 4 days following the second PG 2α injection were then palpated by an ABS route breeder to determine if in estrous. 80% of the group was detected in estrous and submitted to Artificial Insemination after the second PG 2α. Cows not exhibiting signs of estrous continued the program in an attempt to further optimize synchrony. As prostaglandin only regresses the corpus luteum and does not encourage follicular growth, breeding cows not exhibiting estrous would result in suppressed conception rates. For that reason these cows entered the ovsynch segment of the protocol and were then bred at 81 to 88 DIM on the timed A.I schedule regardless if estrous was observed or not. The natural breeding group (control group) was locked daily for estrous detection with tail chalk being the primary method of detection. Cows detected in estrous were then submitted to A.I by an ABS route breeder. The voluntary waiting period was 50 DIM for this group in comparison with the 60 DIM for the experimental group. The control group did not receive any hormones, infusions, or palpations before 100 DIM. If no heat had been reported by 100 DIM, the cow would then be put on the veterinary list and palpated for further examination. Depending on the animal s reproductive condition determined by the veternarian, PG 2α or GnRH might be administered but a systematic breeding program was not administered. Pregnancy diagnosis was performed between 45 and 75 days after A.I by a certified veterinarian. Both groups were diagnosed of pregnancy via rectal palpation of the uterus. As the 10
purpose of this study was to specifically analyze information based off of first time breeding information, a resynchronization protocol was not utilized. 11
RESULTS AND DISCUSSION It is necessary for producers who utilize systematic breeding programs to understand that the effectiveness of these programs relies heavily on the management of the dairy. Strict compliance of the protocol is necessary for any synchronization program to work effectively. When choosing a systematic breeding protocol it is important to understand the practicality, convenience, and effectiveness of each program on the facility. Reproductive management programs vary drastically from farm to farm as every dairy has different goals, management styles, and facilities. Contrary to other studies, the presynch program utilized on this particular herd required more resources and labor than that of the natural breeding group. With an average stocking density of 130%, coupled with the fact of constant non intentional pen movements, locating cows scheduled for treatments was an ongoing challenge. Searching for individual cows amongst a milking heard of 1400 requires an abundance of weekly labor. This can potentially be very time consuming especially if there are numerous cows moved from their original string without notice. For this program to work smoothly on this operation management would need to prevent mixing of pens and reduce overcrowding. In this experiment, a presync ovsync program was compared to a natural breeding program to determine the effectiveness of each. When reviewing the data, one must keep in mind the fact that this study was conducted during the summer months with elevated ambient temperatures. The herd in which the experiment took place did not utilize extensive cooling systems, with shade being the only resource used for cooling. Reproductive performance was negatively impacted as temperatures occasionally surpassed 100 degrees Fahrenheit throughout 12
the study. Although this experiment was not designed to determine the effectiveness of the programs during elevated temperatures, studies show that cows under heat stress have reduced duration and intensity of estrus. The combination of heat stress coupled with overcrowding in breeding pens reduced heat detection efficiency for non synchronized cows. At 120 days in milk (DIM), only 78% of the natural breeding group had been detected for estrous and submitted to AI. The remaining 22% of cows had gone through three estrous cycles without being detected. This could be the cause of heat stress, poor estrous detection or failure of cows to cycle. With a timed AI program the need for estrous detection is eliminated as every cow has a predetermined breeding date. By 82 DIM, 100% of cows in the synchronized program were submitted to AI at least once. While more animals were submitted to AI in the presync group, conception rates for natural services were slightly higher. As shown in previous studies, first service conception rates were lower for synchronized cows; 31% versus 37% for natural services. 38% 36% 34% 32% 30% 28% 1st service conception rates Presync Control Figure 2-1st service conception rates 13
Making up for the lower conception rate is the presynch s 21 day pregnancy rate. Although the natural breeding group had higher conception rates, the presynch group had more pregnant cows (33 vs 28) at 120 DIM. Having more cows pregnant in the earlier stage of lactation resulted with an increase in average pregnancy rates of 19% versus an average of 14% for the control group. 25% 20% 15% 10% 5% 21 Day Preg Rate 0% jul 10-31 Aug 1-22 Aug 22- sep 12 3-Oct Presync Control Figure 3-21 day pregnancy rate 1 This indicates a significant increase in heat detection efficiency with cows concieving in a more timely fashion for the presynch group. This is apparent as the average first service breeding date was 63 DIM for the synchronized group versus 79 DIM for natural breeding groups. 14
Figure 4 DIM 1st Service- presynch group Figure 5 DIM 1st Service- control group *These graphs show the DIM for first service where each segment of the red line represents an individual animal and the horizontal axis represents DIM. It is apparent that the control group s first service is more distributed and contains services in later stages of the lactation. 15
Table 1 Presync Date Presync Data Service of Conception ID Lactation DIM 1st service 1st 2nd 3rd 4th Open >120 DIM 32 1 60 X 34 3 61 X 38 1 61 X 92 1 69 X 116 2 60 X 132 1 53 X 138 2 58 X 154 1 60 X 186 2 80 X 206 1 58 X 254 2 67 X 300 2 60 X 312 1 59 322 3 79 X 328 2 59 X 340 3 59 X 368 2 69 X 402 1 59 X 452 2 79 X 472 4 60 X 690 3 61 X 826 1 69 X 938 2 54 X 4805 2 67 X 5228 1 57 X 8047 2 60 X 8072 2 58 X 8194 3 60 8246 2 79 X 8436 1 59 X 8554 3 57 X 8604 2 65 X 8650 2 60 X 8668 3 79 X 8914 2 58 X 8920 2 58 X 8924 2 61 X 8926 1 60 X 8950 1 59 X 8974 3 64 X 8982 3 61 X 9040 4 59 X 9420 4 53 X # Av Lact DIM 1st service 1st 2nd 3rd 4th %open 43 2.05 62.51 30.23% 23.20% 11.60% 2.30% 27.90% 16
Table 2- Control Data Control Data Service of Conception ID Lactation DIM 1st service 1st 2nd 3rd 4th Open >120 DIM 31 1 55 X 35 1 58 X 61 2 76 201 1 105 X 215 3 82 X 219 2 X 263 1 X 301 4 85 X 317 3 59 X 383 2 X 395 1 61 X 499 1 95 X 649 3 61 X 659 3 63 X 681 2 133 X 695 3 80 X 723 2 59 X 881 1 86 X 905 2 X 1179 2 120 X 2009 3 121 X 6365 2 60 X 7815 2 X 7843 2 64 X 7851 1 57 X 8085 4 X 8281 3 122 X 8317 2 56 X 8487 2 71 X 8585 2 X 8589 1 80 X 8661 1 60 X 8677 2 55 X 8927 2 66 X 8933 4 X 9011 2 X 9029 3 75 X 9055 1 132 X 9079 2 70 X 9107 1 90 X 9401 4 X 9455 3 105 X 9457 2 56 X # Av Lact DIM 1st service 1st 2nd 3rd 4th %open 43 2.12 79.33 37.20% 13.95% 6.98% 4.65% 34.80% 17
CONCLUSION As this study was limited to analyzing first service post partum conception rates and pregnancy rates up to 120 DIM, there is room for further analyzes with a resync program throughout the duration of a lactation. Analysis of average days open and average DIM would be measurements to further support evidence of success if a complete lactation was monitored. Larger populations within groups studied would also support a more accurate conclusion. Although this study was conducted on a relatively small population and results were only monitored up to 120 DIM, there is still evidence of reproductive improvement with the implementation with of an estrous synchronization program. With both breeding programs having their pros and cons, it is the 21 day pregnancy rate which truly determines the success of a reproductive program. The presynchronization program offered a 4% increase in the pregnancy rate, which will have a positive impact on many aspects of the dairy. A computer stimulation from the University of Florida indicates that by every percent increase in pregnancy rate is a return of $14.49 per cow/year (De Vries et. al, 2009) {4 points increase X (14.49 X 1400)}=$81,144. The computer program assumed milk was at $16/cwt which was close to the price during the study. Contributing factors allowing for an extra $81,144 are lower cull rates, increase number of replacements, and higher milk production. The costs associated with the increased pregnancy rates for this study were drug cost and labor. Drug costs for the administration of this specific presynchronization 18
program consist of PG 2α and GnRH which are $2.8/dose and $3/dose respectively. Together these combine for a total of $7.24/cow. If all cows remained on the presynchronization program for the duration of the protocol rather than picking cows off of the second lutalyse injection, the cost per cow would be $ 14.40/cow. Labor associated with the implementation of the synchronization program was $2.79/cow at $10/hr. For both labor and drug expenses the combined total per cow was $10.03. According to the University of Florida s computer stimulation the synchronization program would provide a net return $4.46/cow ( $14.49-10.03). After analyzing results from both programs, the most effective breeding program might be a combination of both approaches. With the natural breeding group having higher first service conception rates, it might be reasonable to detect natural heats for the cow s first service. This approach would significantly reduce labor and drug cost. Cows entering over 80 DIM without a detected estrous would then be placed on a synchronization program to induce estrous before entering later DIM without submission to A.I. This approach might not yield an equally high pregnancy rate but would be a more cost effective method to improving the current pregnancy rate. 19
REFERENCES ABS Global. 2008. A.I Management Manual. Vol. I. 6 th ed. ABS Global, Inc. DeForest, Wisconsin. Bilby, R.T., and R.C. Chebel.2009. Back to basics: Heat detection is still the best tool for getting cows pregnant.western Dairy News. Vol. 9, No. 1 De Vries. A., J. van Leeuwen and W.W. Thatcher. 2009. Economics of Improved Reproductive Performance in Dairy Cattle. University of Florida IFAS Extension. Gumen, A., R. Sartori, H. Lopez and A. Souza. 2006. Management and treatment of anestrous and anovulatory conditions in dairy cattle. Departement of Dairy Science University of Wisconsin- Madison. Hutchinson, J. L. 2008. Troubleshooting infertility problems in dairy cattle. Milkproduciton.com Olynk, N.J. and C.A. Wolf. 2008. Economic Analysis of Reproductive Management Strategies on US Commercial Dairy Farms. J. Dairy Sci. 91:4082 4091doi:10.3168/jds.2007-0858 Rabiee, A. R., I.J. Lean, and M.A. Stevenson. 2005. Efficacy of Ovsynch Program on Reproductive Performance in Dairy Cattle: A Meta-Analysis. J. Dairy Sci. 88: 2754-2270 Smith, R.D.,and L.E. Chase.2010. Nutrition and Reproduction. Dairy Integrated Reproductive Management. Smith, R.D., and L.E. Chase. 2010. Factors affecting conception rates. Dairy Integrated Reproductive Management. Thatcher, W. W., D.J. Patterson, F. Moreira, M. Pancarci, and E. R. Jordan. 2001a. Current concepts for estrous synchronization and timed insemination. 34 th Ann. Proc. Am. Bovine Prac. 34:96-105. Thorson, S. 2006. What Synchroniation Program is Right For You? Genex Cooperative, Inc. Wattiaux, M.A. 2010. Dairy Essentials. Ch. 9 Reproduction and Genetic Selection. The Babcock Institute for International Dairy Research and Development. 20
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